New Efficient Electrocatalyst for the Hydrogen Evolution Reaction

Apr 17, 2017 - Simultaneously, density functional theory (DFT) computations proved ... A New Platinum-Like Efficient Electrocatalyst for Hydrogen Evol...
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New Efficient Electrocatalyst for the Hydrogen Evolution Reaction: Erecting a V2Se9@Poly(3,4ethylenedioxythiophene) Nanosheet Array with a Specific Active Facet Exposed Haitao Xu,† Zhiqiang Jiang,‡ Huijuan Zhang,† Li Liu,† Ling Fang,† Xiao Gu,*,‡ and Yu Wang*,† †

The State Key Laboratory of Mechanical Transmissions and the School of Chemistry and Chemical Engineering, and ‡Department of Applied Physics, Chongqing University, 55 Daxuecheng South Road, Shapingba District, Chongqing City, People’s Republic of China 401331 S Supporting Information *

ABSTRACT: To obtain catalysts with remarkable activity for the hydrogen evolution reaction (HER), rational design and synthesis of catalysts with rich active sites are very urgent. Herein, we reported, for the first time, V2Se9 nanosheet arrays exposed with the highly active (100) facet as a new efficient catalyst for HER. The highly active but thermodynamically instable (100) facet was converted from V2O5 based on a low crystal-mismatch strategy. Furthermore, conductive poly(3,4ethylenedioxythiophene) (PEDOT) acting as a co-catalyst further contributed to the redistribution of charge and reduction of hydrogen adsorption energy. Due to the strong synergistic effect between V2Se9 and PEDOT, the resulting material, noted as V2Se9@PEDOT NSs/NF, exhibited excellent electrocatalytic performance among selenide catalysts, for example, a small overpotential of 72 mV at 10 mA cm−2, a low Tafel slope of 36.5 mV dec−1, and remarkable durability. Simultaneously, density functional theory (DFT) computations proved that the adsorption free energy of H* (ΔGH*) for V2Se9@PEDOT NSs/NF (0.09 eV) is comparable to that of Pt (around 0.09 eV).

H

V2Se9, VSe2, VS2, VS4) have been rarely reported despite their potential to catalyze the HER. Recently, Pan et al. and Wang et al. first proved the high catalytic activity of VS2 toward HER based on theory and experiments, respectively.5,6 In this context, it is worth wondering whether V2Se9, one of the scarcely reported TMDs, delivers high catalytic activity toward HER. Proverbially, broad active sites and good electric properties are two key factors for HER. Previous studies have shown that the rational design of highly catalytically active crystallographic facets with a high exposing ratio is one of the most promising routes to improve their catalytic activity and durability.7,8 Besides, considering that 2D catalysts are famous for their large specific area and broad electrochemical active sites, it is motivating to rationally design and regulate the active sites on 2D specific crystal facets to release the potential catalytic ability.9 On the other hand, compound catalysts are well-known

ighly active (100) exposed V2Se9 nanosheet arrays were obtained based on a low crystal-mismatch strategy. Then, conductive poly(3,4-ethylenedioxythiophene) (PEDOT) acting as a co-catalyst was deposited onto the surfaces of V2Se9 nanosheets. Due to the strong synergistic effect between V2Se9 and PEDOT, the resultant structures showed excellent electrocatalytic activity and durability toward hydrogen evolution. Hydrogen has been broadly considered as an ideal candidate for clean energy due to its high energy density and zero emission. Water electrolysis acts as one of the emerging green technologies to produce hydrogen with high quality. As is wellknown, state-of-art Pt-based catalysts for hydrogen evolution reaction (HER) exhibit a low overpotential, a small Tafel slope, and excellent stability. However, due to their high cost and low earth abundance, the pursuit of low-cost, high-efficiency catalysts for HER has been unmet.1,2 For this purpose, a series of HER catalysts based on non-noble metals, such as MoS2, WS2, NiSe2, CoSe2, and FeSe2, have been developed.3 Among various HER catalysts, transition metal dichalcogenides (TMDs) have attracted much attention for their high catalytic activity and low cost.4 However, some specific TMDs (e.g., © XXXX American Chemical Society

Received: March 8, 2017 Accepted: April 17, 2017 Published: April 17, 2017 1099

DOI: 10.1021/acsenergylett.7b00209 ACS Energy Lett. 2017, 2, 1099−1104

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http://pubs.acs.org/journal/aelccp

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Figure 1. (a) Fabrication process of V2Se9@PEDOT NSs/NF. (b) FE-SEM images of V2Se9 NSs/NF. (c) Low- and (d) high-magnification FESEM images of V2Se9@PEDOT NSs/NF.

for their synergistic effect, producing “a whole greater than the sum of the parts”. Generally, PEDOT is admired as a highperformance co-catalyst in electrocatalysis, on account of its high chemical stability, electrical conductivity, and catalytic durability.10−12 Hence, it is reasonably worth expecting that the PEDOT-coated V2Se9 NSs/NF with the high-activity facet exposed should possess good conductivity and broad active sites in improving the HER performances. To directly understand the intrinsic character and electrocatalytic activity of different facets of exposed V2Se9 nanosheets, taking (010), (100), and (001) as examples, DFT computations were carried out by contracting corresponding theoretical models (see the Theoretical Section in the Supporting Information for further details). The surface energies of different surface terminations indicate that the relative stability of the three planes is on the order of (010) > (001) > (100); thus, the (100) facet is the most energetically active one (Table S1). Generally, ΔGH* usually acts as an evaluation indicator of the electrocatalytic activity toward HER, and the optimal ΔGH* should approach 0 eV.13 It is verified that the ΔGH* value of the (100) facet is the smallest, in contrast with the other two facets, manifesting that the (100) facet possesses the highest electrocatalytic activity (Table S2). Unfortunately, due to the thermodynamic instability of the (100) facet, it is difficult to directly grow a V2Se9 nanosheet exposed with the (100) facet. The fabrication strategy is illustrated in Figure 1a. The typical scanning electron microscopy (SEM) images and X-ray diffraction (XRD) pattern of NH4VO3 and V2O5 are displayed in Figures S1 and S2.14 As seen in Figure 1b, the nanosheets are nearly transparent under electron irradiation, indicating their ultrathin feature. Furthermore, the energy-dispersive X-ray (EDX) line profile and inductively coupled plasma optical emission spectroscopy (ICP) data verify the composition of obtained V2Se9 (Figure S3). SEM images of the V2Se9@ PEDOT NSs/NF nanostructure with various magnifications are displayed in Figure 1c,d. It is obvious that porous PEDOT was uniformly coated on the surface of V2Se9 nanosheets. Figure 2a shows the crystal structures of V2Se9, and the density of states (DOS) in Figure 2b shows that bulk V2Se9 has an energy gap of

Figure 2. (a) Structure of V2Se9; red and gray spheres denote V and Se atoms, respectively. (b) DOS of bulk V2Se9; the Fermi energy is set to zero. (c) XRD pattern of the V2Se9@PEDOT NSs/NF (the peaks denoted by asterisks originating from the NF substrate). (d) FTIR spectrum of the EDOT monomer and PEDOT.

0.3 eV, which indicates that this material may have good electric conductivity. As seen in Figure 2c, all of the diffraction peaks, except for two peaks that stemmed from NF, could be indexed to the monoclinic V2Se9 (JCPDS No. 77-1406), which further confirms the successful synthesis of V2Se9 crystals. No diffraction peaks of PEDOT layers are visible in the XRD patterns, manifesting the amorphous feature of PEDOT shells. Besides, Figure 2d delivers the typical Fourier transform infrared (FTIR) spectra of PEDOT. The peaks at 980 and 840 cm−1 are indexed to the C−S stretching vibration, and the peak at 1642 cm−1 is attributed to the doped level of PEDOT.15,16 In order to determine the core−shell structure and exposing crystal face of V2Se9@PEDOT NSs/NF, the transmission electron microscopy (TEM) and high-resolution (HR) TEM images of V2Se9@PEDOT NSs are analyzed. TEM images in Figure 3a,b confirm the nanoporous structures of V2Se9@ 1100

DOI: 10.1021/acsenergylett.7b00209 ACS Energy Lett. 2017, 2, 1099−1104

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Figure 3. (a) Low- and (b) high-magnification TEM images of V2Se9@PEDOT nanosheets. (c−f) EELS mapping of V, Se, and C in V2Se9@ PEDOT NSs. HRTEM images of V2O5 (g) and V2Se9 (h) nanosheets. Insets are the corresponding FFT patterns.

Figure 4. (a) XPS spectra of S 2p in V2Se9@PEDOT NSs/NF and PEDOT/NF; XPS spectra of V 2p1/2 (b), V 2p3/2 (c), and Se 2p (d) in V2Se9@PEDOT NSs/NF and V2Se9 NSs/NF, respectively. (e) Electron charge density difference profile of PEDOT@V2Se9; the yellow color area donates the isosurface level of 6.7 × 10−3 Å−3. (f) HER free energy change for PEDOT, V2Se9, and PEDOT@V2Se9. The comparison of different materials is taken from ref 16.

(020) and (210) crystal planes of the orthorhombic V2O5 phase. Next, the designed V2O5 NSs/NF was converted into the V2Se9 NSs/NF after the recrystallization and deoxidation process. Two sets of well-resolved crystal lattices are observed in Figure 3h, possessing two interplanar spacings of 0.20 and 0.31 nm, respectively, corresponding to the (004) and (040) crystal planes of the monoclinic V2Se9 phase.19 It is worth noting that the crystal mismatch (atom arrangement) between V2O5 and V2Se9 is below 10% based on the crystal planes of (004) and (040) in V2Se9 and (020) and (210) in V2O5. Moreover, both of the corresponding interplanar angles are exactly 90° for V2O5 and V2Se9. As a result, the thermodynamic instability facet of (100) was successfully obtained for the first time without any necessity to conquer the large energy gap. X-ray photoelectron spectroscopy (XPS) was performed to discover the synergistic effects between V2Se9 and PEDOT. It is noted that the V, Se, and S originated from V2Se9 and PEDOT. As displayed in Figure 4a, all of the S peaks of V2Se9@PEDOT NSs/NF are positively shifted to higher binding energy with

PEDOT NSs, providing effective transport channels for the electrolyte during the electrocatalysis process. Nitrogen adsorption−desorption isotherms were conducted to further verify the porous structure and specific surface areas of the V2Se9@PEDOT NSs. The specific surface areas are calculated to be 53 m2 g−1, and mesopores mainly concentrate at 2, 6, and 12 nm (Figure S4). To identify the core−shell nanostructure and uniformity of the PEDOT coating, electron energy loss spectroscopy (EELS) mapping was conducted and is displayed in Figure 3c−f. The results indicate that V and Se are welldistributed in the inner V2Se9 crystals and the architecture possesses an obvious PEDOT shell with a thickness of 6 nm. Remarkably, due to the thermodynamic instability of the (100) facet, a low crystal-mismatch strategy was adopted to designedly fabricate V2Se9 NSs/NF with exposed (100) facets.17,18 First, we synthesized V2O5 NSs/NF with exposure of a specific facet that has similar atom arrangements as V2Se9. As seen in Figure 3g, the crystal lattice spacings are determined to be 0.18 and 0.30 nm, respectively, corresponding to the 1101

DOI: 10.1021/acsenergylett.7b00209 ACS Energy Lett. 2017, 2, 1099−1104

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Figure 5. LSV curves of (a) V2Se9@PEDOT NSs/NF, V2Se9 NSs/NF, PEDOT/NF, and NF. Tafel plots of (b) V2Se9@PEDOT NSs/NF, V2Se9 NSs/NF, and PEDOT/NF. (c) Long-term durability test conducted at a constant current density of −10 mA cm−2. TEM images of V2Se9@ PEDOT nanosheets with deposition times of 200 (d), 500 (e), and 900 s (f). (g) LSV curves of V2Se9@PEDOT NSs/NF with different coating thicknesses. (h) Nyquist plots of V2Se9@PEDOT NSs/NF and V2Se9 NSs/NF.

that of NF, PEDOT/NF, and V2Se9 NSs/NF. Moreover, the V2Se9@PEDOT NSs/NF, V2Se9 NSs/NF, and PEDOT NSs/ NF respectively require 72, 91, and 234 mV to afford a current density of 10 mV cm−2, indicating that NF has no direct contribution to the high catalytic activity. In addition, the overpotential of V2Se9@PEDOT NSs/NF is among the most active substrate-based HER catalysts in acidic media (Table S3). The Tafel slopes of V2Se9@PEDOT NSs/NF, V2Se9 NSs/ NF, and PEDOT NSs/NF were 36.5, 47, and 103 mV dec−1, respectively (Figure 5b). It is worth noting that such a high catalytic activity in acidic media has never been achieved for selenide catalysts before, such as FeSe2,24 CoSe2,25 NiSe2,26 MoS2,27 and WS2(1−x)Se2x.28 Remarkably, the outstanding catalytic activity of V2Se9@PEDOT NSs/NF results from the redistribution of charge and depressed hydrogen adsorption energy. We further evaluated the long-term durability of V2Se9@PEDOT NSs/NF via chronopotentiometric measurement. Figure 5c demonstrates that the overpotential of V2Se9@ PEDOT NSs/NF only increases 1.5% after a 25 h test. The outstanding electrocatalytic durability of V2Se9@PEDOT NSs/ NF is further confirmed by polarization curves and SEM images after 5000 cycles. (Figure S5a,b). Because the thickness of PEDOT layers could be controlled via deposition time, it is necessary to evaluate its effect on electrocatalytic activity (Figure 5d−f). The polarization curves of V2Se9@PEDOT NSs/NF with various coating thicknesses of the PEDOT layer are displayed in Figure 5g. When the coating thickness is 6 nm, the structure possesses the highest electrocatalytic activity. The crystal face effect of V2Se9 NSs/NF is further supported by evidence that the catalytic activity of V2Se9 NSs/NF exposed with the (100) facet is better than that of V2Se9 NSs/NF exposed with other facets (Figures S6 and S7). To explore the electrochemical reason behind the excellent electrocatalytic activity of V2Se9@PEDOT NSs/NF, the electronic conductivity and electrochemical double-layer capacitance (Cdl) were determined via electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) curves. The EIS data were fitted with a simplified equivalent circuit (Figure S8). Figure 5h and corresponding parameters of the equivalent circuit (Table S4) reveal that the electronic resistivity of V2Se9@PEDOT NSs/NF is smaller than that of V2Se9 NSs/NF due to the coated conductive PEDOT layers. Furthermore, Cdl was estimated from CV curves (Figure S9).

respect to those of PEDOT/NF, indicating the existence of strong electronic interaction between V2Se9 and PEDOT. Besides, the Se 2p, V 2p3/2, and V 2p1/2 peaks of V2Se9@ PEDOT shift to a lower binding energy, providing further evidence that intense electronic interaction involves V2Se9 in PEDOT(Figure 4b−d). The underlying causes for the electronic interaction are explained by the electronic charge density difference in Figure 4e. By performing the Bader charge analysis, we could estimate the magnitude of charge transfer in PEDOT@V2Se9. The sulfur atom in PEDOT would lose 0.14 e−, while the upper vanadium atom would receive 0.06 e− in the PEDOT@V2Se9 composite compared to their charge value in PEDOT and V2Se9. As a result, the S atoms carry more positive charge, whereas the V atoms carry more negative charge in PEDOT@V2Se9, in contrast with their charge states in PEDOT and V2Se9. Generally, the intense electronic interactions between V2Se9 and PEDOT could change the electronic state of V2Se9, which in turn enhances the catalytic performances of V2Se9@PEDOT NSs/NF.20 The synergistic effects were further proved by DFT calculations. The process of HER can be summarized in a three-state diagram, consisting of an initial catalyst−water state, an intermediate catalyst−H atom state, and a final catalyst−H2 state as the product. The favorable ΔGH* of V2Se9 NSs/NF (0.19 eV) is much smaller than those of PEDOT (2.68 eV) and V2Se9 (0.191 eV). Noteworthily, the vast difference of ΔGH* between PEDOT and the V2Se9 crystal indicates that V2Se9 guarantees much lower energy for adsorbing on active sites compare with that of PEDOT.21,22 As a results, protons in electrolyte would first penetrate the porous PEDOT film and then adsorb on electronegative active sites on V2Se9 crystals. Surprisingly, the ΔGH* of V2Se9@ PEDOT NSs/NF is 0.09 eV, which is even comparable to that of Pt (around 0.09 eV).23 In other words, due to a series of synergistic effects between V2Se9 and PEDOT, the hybrid nanomaterials should possess excellent catalytic activity toward HER. After proving the synergistic and crystal plane effects by theoretical simulation, we studied the actual electrocatalytic activity of V2Se9@PEDOT NSs/NF with a mass loading of 1.7 mg cm−2. The polarization curves in Figure 5a demonstrate that V2Se9@PEDOT NSs/NF possesses near-zero onset overpotential, and with the overpotential decreasing further, the cathodic current rapidly increases, which is much better than 1102

DOI: 10.1021/acsenergylett.7b00209 ACS Energy Lett. 2017, 2, 1099−1104

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ACS Energy Letters The Cdl of V2Se9@PEDOT NSs/NF (11.6 mF cm−2) is much higher than that of V2Se9 NSs/NF (5.2 mF cm−2). However, the PEDOT polymer could block some of the active sites, but the currents of V2Se9@PEDOT NSs/NF for HER are higher compared to those of V2Se9 NSs/NF, denoting that the better performance of V2Se9@PEDOT is not due to more actives sites. Hence, the improved electrocatalytic performances might be due to higher stability of the active sites, much lower energy for adsorbing on active sites, and smaller electronic resistivity. In other words, the covered active sites could be made up by other positive advantages induced by PEDOT. In conclusion, we developed a novel, binder-free, non-noblemetal catalyst-based PEDOT-coated V2Se9 NS, with an exposed (100) facet, for the first time. To the best of our knowledge, the V2Se9@PEDOT NSs/NF exhibited the best electrocatalytic performances among selenide catalysts. The outstanding electrocatalytic activity of V2Se9@PEDOT NSs/NF is ascribed to the following five aspects: (I) The hierarchical open-ended architecture provides numerous catalytically active sites for catalytic reactions. (II) The close-knit contact between V2Se9@ PEDOT NSs and NF speeds up the electronic transmission, contributing to the improvement of V2Se9@PEDOT NSs/NF’s capability. (III) V2Se9 NSs/NF, exposed with the (100) facet, could provide more catalytically accessible surface atomic sites, which results in rapid charge transmission and beneficial energetics for HER. (IV) The porous PEDOT coating could not only lead to lower energy for adsorbing on active sites but also improve the electrical conductivity of the architecture. Thus, all of the catalytically active sites are more easily accessible to electrons coming from the electrode. (V) The improved electrocatalytic performances of V2Se9@PEDOT NSs/NF are primarily due to the synergistic effect of the unique V2Se9−PEDOT hybrid architecture, which may result in redistribution of the charge and lower the adsorption energy and accordingly benefit the HER. All of the above-mentioned merits make it a promising alternative to Pt for robust electrochemical hydrogen evolution.

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Fundamental Research Funds for the Central Universities (0301005202017), the Beijing National Laboratory for Molecular Sciences (BNLMS), and the Hundred Talents Program at Chongqing University (Grant No. 0903005203205).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00209. Experimental details, theoretical calculation, SEM, EDX, BET data, electrochemical data, and more results to conform the excellent electrocatalytic activity (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.W.). *E-mail: [email protected] (X.G.). ORCID

Yu Wang: 0000-0003-2883-1087 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Thousand Young Talents Program of the Chinese Central Government (Grant No. 0220002102003), the National Natural Science Foundation of China (NSFC, Grant No. 21373280, 21403019), the 1103

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DOI: 10.1021/acsenergylett.7b00209 ACS Energy Lett. 2017, 2, 1099−1104